Continue to troubleshoot or start from scratch?

i don't know about darts, but 74 dusters was 1st year mopar put a seat belt interlocking module, its a ivory colored plastic module next to fuse panel, it just unplugs, you need to check it for corrosion, mine was wet and corroded, you can't just cut it out, it controls 11 different things ,from dome light, dash light, turn signal, parking lights and several other things, and under the hood next to the washer bottle there is a reset button with 3 yellow wires that has to be by passed if you don't have wire going under seat that detects someone sitting in it. They have a green module and a orange , one from Canada and one from cars in Venezuela ,because they had no seat belt laws,and are bypass modules, but on ebay they go for about $150 . After 6 months trying to get my duster to start, i finally stripped all the under dash wiring and bought all the under dash wire from a 71 dodge dart and got rid of module, only $50 for wiring, then i rewired the reset button on the fender well under the hood and done away with the reset button. The modules caused so much trouble that Mopar quit using them in 1975. may not be your trouble but will give you an idea should you have the ivory module in your wiring harness. By the way i was doing a 6 to 318, so hope this gives you a clue, mine would start from crossing at relay like yours, but could never get the key to work until i change wiring and if you have 2 plugs hanging down , it could be that previous owner unplugged interlocking seat belt module, i found 8 or 10 ivory ones, but if you take the plastic off the modules most are white where moisture has taken a toll on them sitting for 20 and 30 years